depfile: variable with all identifiers (#34678)

With the new variable [prefix/]SPACK_PACKAGE_IDS you can conveniently execute
things after each successful install.

For example push just-built packages to a buildcache

```
SPACK ?= spack
export SPACK_COLOR = always
MAKEFLAGS += -Orecurse
MY_BUILDCACHE := $(CURDIR)/cache

.PHONY: all clean

all: push

ifeq (,$(filter clean,$(MAKECMDGOALS)))
include env.mk
endif

# the relevant part: push has *all* example/push/<pkg identifier> as prereqs
push: $(addprefix example/push/,$(example/SPACK_PACKAGE_IDS))
	$(SPACK) -e . buildcache update-index --directory $(MY_BUILDCACHE)
	$(info Pushed everything, yay!)

# and each example/push/<pkg identifier> has the install target as prereq,
# and the body can use target local $(HASH) and $(SPEC) variables to do
# things, such as pushing to a build cache
example/push/%: example/install/%
	@mkdir -p $(dir $@)
	$(SPACK) -e . buildcache create --allow-root --only=package --unsigned --directory $(MY_BUILDCACHE) /$(HASH) # push $(SPEC)
	@touch $@

spack.lock: spack.yaml
	$(SPACK) -e . concretize -f

env.mk: spack.lock
	$(SPACK) -e . env depfile -o $@ --make-target-prefix example

clean:
	rm -rf spack.lock env.mk example/
``

share/spack/templates/depfile/Makefile

@@ -1,6 +1,9 @@
 SPACK ?= spack
 SPACK_INSTALL_FLAGS ?=
 
+# This variable can be used to add post install hooks
+{{ pkg_ids_variable }} := {{ pkg_ids }}
+
 .PHONY: {{ all_target }} {{ clean_target }}
 
 {{ all_target }}: {{ env_target }}